SMITHS "servomotor" 1111SCM-2
 

Hi all,

I'm looking for help please with figuring out a bit of an unusual motor from an attitude indicator form an English Electric Lightning aircraft, there are several of these motors inside it.

Manufacturer: SMITHS
Type: 1111SCM-2
Stores ref:6TD/4551218

The Reference coil is 115Vac, and the Control coils are 60Vac, all 400Hz.

Regarding the "SCM" in the type, I'm tempted to guess it stands for "Servo Control Motor", but this is an assumption, and you know what they say about assumptions...

I'm told that in similar aircraft instruments it is normal to see quadrature phase motors with two windings, marked Ref and Control, which should have a 90 degree phase angle between applied voltages. In normal quadrature phase motors, the Reference coil has a constant AC signal applied, but the amplitude of the Control coil sets the motor speed, and by giving the Control coil a -90 degrees phase angle from Ref, reverses direction of rotation. This info was from an ex air force friend who has a vast experience in aircraft instrumentation, but he's not seen a motor like this with two control coils, he's used to seeing one.

My confusion here is that these motors have two Control windings, and I don't quite know what to do with them.

There is one clue in the instrument wiring, the two Control coils have one end of each connected together, so the two Control coils are connected in series, with three wires back to the control circuits (you can visualise it a bit like a centre-tapped transformer wiring).

Powering the Ref coil is straightforward, 115Vac 400Hz, but what do I do with the two control coils?

Just thinking out loud, and guessing, could one Control coil be for clockwise, the other for anticlockwise? Ie: could these motors be for applications where the control circuit can't flip the control signal through 180degrees to achieve -90degrees for reverse direction? ie: inside the motor they could be arranged opposite to each other.

Any ideas, experiences, datasheets, user manuals, other info, will be very much appreciated as I'm currently flying blind with reverse-engineering them. Thought about approaching Smiths, but I assume they will probably either not be interested, nor have records dating back to the early 60's.

Many thanks, Scott.

Re: SMITHS "servomotor" 1111SCM-2
 

Are you sure it is not a Resolver, rather than an AC motor?

Ron

Re: SMITHS "servomotor" 1111SCM-2
 

There are two different systems in use for these sorts of things

'Synchros' work on a three-phase system

and 'Resolvers' work on a two phase system.

As key words, you'll find a lot on each in wikipedia etc. and you'll need to do some background reading on each.

The systems aren't necessarily servos, because servos are normally taken to mean the presence of motors with feedback systems. Synchros and resolvers can be used in the feedback arrangement of a servo system, but they are also used open-loop as usefully accurate indicators.

If you contacted Smiths, there is the possibility that you might get passed to someone who would help, but most likely in any large formal large business, it would come to official attention and someone would have to estimate the time and cost to formulate a reply. After smelling salts had been administered to the accountants, a terribly nicely written 'terribly sorry, but' reply would be written. The following month would be taken by people calculating the cost of that reply, and by people calculating the cost of that calculation.... ad inf.

Anyway, a lot of what you need to know is out there and freely available. It'll be in general terms rather than specific to your unit, so you first need to understand the general systems, and then see how your unit fits in.

Analog Devices make synchro/resolver to digital interface chips at prices that will scare you.

https://en.wikipedia.org/wiki/Synchro
https://en.wikipedia.org/wiki/Resolver_(electrical)

The two systems are rotary transformers using a polyphase connection so that direction is not ambiguous. Rotating one causes others to move in sympathy. Accuracy down to a degree is possible so they get used in aircraft for relaying the movements of gyro compasses, attitude gyros, flap positions etc. Warships used them for gun laying, compass repeaters etc.

Your two control windings suggest it is a 'resolver' rather than a synchro, but a lot of the understanding of resolvers is explained as an addition to synchros which came first. Read both, it will be clearer.

david

Re: SMITHS "servomotor" 1111SCM-2
 

Hi chaps, thanks for the replies and help...

I can see what you're thinking as I initially did think the same.... however, I am convinced that these are motors, not synchros or resolvers / RVDT (even though they look almost identical at a glance).

The main reasons for this conviction are:


- They drive a gear-train that has a massive gear ratio reduction before getting to the indicator, so it has to rotate really quite fast to move the indicator.

- They state a torque and max speed on the markings.

- The same gear-train also has a synchro control transmitter attached at the other end to feed back the indicator position to the control circuitry, meaning they must be the motor to drive the gear-train.

- They are called "Servomotors" on the markings.


I can find online information on quadrature phase motors, but only with a Ref and single Control coil, nothing for motors with two Control coils.

With the feedback synchros, I have in my head a software concept for a way to read the angles, still to try it, but that's another topic.

Cheers, Scott.

Re: SMITHS "servomotor" 1111SCM-2
 

There is quite a good chapter about Servomotors in Muirhead's "Synchro Engineering Handbook" by Upson and Batchelor. I've attached a (hopefully readable) photo of a couple of pages . If you look at the last part of the last paragraph of page 72, it does refer to the use of Servomotors with amplifiers, with the two control windings joined in series or parallel.

I can probably scan the whole chapter if you are interested.

Ron

Re: SMITHS "servomotor" 1111SCM-2
 

Hi Ron,

That paragraph really does sound exactly like these motors, so according to the book it's a case of using the windings in series applying 120V across the pair and treat it like a single coil. I won't necessarily need the centre tap, but its an option depending on the control circuit I develop for it.

Many thanks for that info!! Much appreciated!

If you wouldn't mind scanning that chapter, I'd be most grateful for the help!! when i eventually write this project up on my website, I'll include a thank-you note! I've looked on ebay and amazon and can't find a copy of the book...

Cheers, Scott.

Re: SMITHS "servomotor" 1111SCM-2
 

There is also in Upson and Batchelor's book a diagram of a "Simple AC position Servo" in another chapter which might indicate the system that your servomotor drives. Feedback from the synchro Control Transformer drives the Servomotor amplifier input.
This may help.

Ron

Re: SMITHS "servomotor" 1111SCM-2
 

Brilliant Ron thanks, that's exactly the type of system I'm dealing with!

But I'll be interfacing with the motor and synchro directly.

Re: SMITHS "servomotor" 1111SCM-2
 

I have had a play with a two coil unit.
It is a rotation sensor with two coils.
There is an exciter coil and an output coil that is in phase with the exciter in one direction and out of phase in the other with amplitude for angle for 90 degrees in each direction.
The exciter voltage is much lower than your unit as marked. It is something like 12 or 24 volts.
It looks externally like yours.

Re: SMITHS "servomotor" 1111SCM-2
 

Hi, the more I discover about synchros, resolvers, RVDT's, and now this motor, the more I find there are a lot of very similar appearing devices with subtle differences!

I'm sure this is a motor, it has to be. Based on Ron's previously posted book photos, I just now need to try and operate it! Will start out on lower voltage (and hence power) with no load to prove the theory without damaging it before going to 115Vac.

Cheers, Scott.

Re: SMITHS "servomotor" 1111SCM-2
 

I think the external parts were the same for many types with a variety of functions.

Re: SMITHS "servomotor" 1111SCM-2
 

Scott

It was common practice on test benches to use a 100V line power amplifier and an AF oscillator to generate 115V 400Hz reference supplies for Synchros etc. Do you plan to do something similar?

Ron

Re: SMITHS "servomotor" 1111SCM-2
 

Definitely a motor, two coils so can series or parallel connect them, depends if you have a valve, high impedance, or transistor, low impedance, servo amplifier.

Sure I have a copy of the Muirhead book, somewhere. I had hundreds of synchros etc and used to sell them, but all of a sudden no one wanted them so scrapped. I would point out that the brushes a re what is known as precious metal, a gold alloy, about £600 per oz.

Just scrapped the test angle unit, might have a 400Hz supply if wanted.

Re: SMITHS "servomotor" 1111SCM-2
 

Hi, I have an ex-aircraft 400hz 3 phase rotary inverter (28v DC supply), thanks though.

But in my application here, I will try generating my own rough sine waves using an Arduino, opto isolation and H bridge PNP/NPN complimentary pair, on big heat sinks.

Cheers, Scott

Re: SMITHS "servomotor" 1111SCM-2
 

That's not really the way it worked at Smiths. The "official attention" would probably have come from my desk, and I'd have done my best to help. I'd have known one or two senior avionics design engineers who'd have been equally enthusiastic about researching a bit of technical background. No accountants would have been harmed in the process!

Smiths Aerospace, however was sold to General Electric in 2007, so my recollections are unfortunately of little use . A very long shot would be to contact the original Smiths plant at Bishops Cleeve, Cheltenham (now GE Aviation); tel 01242 673333. Ask for customer support and hope you're put through to someone helpful. Your synchro was probably manufactured at Bishops Cleeve, but any knowledge has probably now evaporated due to staff retirements.

An alternative might be to contact the Imperial War Museum. For example, see https://www.iwm.org.uk/collections/item/object/70000160 . You may well come across a knowledgable avionics enthusiast - you have the advantage that aviation engineers do rather enjoy their subject!

Best of luck.

Martin

Re: SMITHS "servomotor" 1111SCM-2
 

Hi Martin, thank you for that. As you say, there won't be many employees left there who know these devices and systems, let alone having any documentation still stashed away.

I used to work in a big company, Rolls-Royce, on the industrial engines, and really, anything older than 15 years we struggled for information. When I was there (left about 6 years ago) they had stopped supporting the Olympus and Proteus, but continued with supporting the Avon simply as there were so many still in service. They were selling new RB211 and Trent units.

Re: SMITHS "servomotor" 1111SCM-2
 

Hi Scott, you might also like to look up the "Scott connection" it is a means of generating a 2 phase supply from a 3 phase supply. These were often used for generating references and converting between Polar and Cartesian co ordinates.
Lots of stuff on the net about these


Ed

Re: SMITHS "servomotor" 1111SCM-2
 

Hi Ed,

In this crazy historic world of AC signalling, the Scott-T transformer is still one of the things that I don't quite understand how it does what it does! I get that it can take a 3 phase Synchro signal and turn it into a 2 phase RVDT / LVDT type signal, which these days is easier to turn into something useful with commonly available converters.

Luckily I had a lot of help from an friend in understanding the various types of synchro, he used to give training on aircraft instrumentation and radios.

Cheers, Scott

Re: SMITHS "servomotor" 1111SCM-2
 

Hi Scott, there were also a lot of hybrid "bricks" around that were used to convert various types of servo signals into other types; usually quite cheap to pick up as few people know what they are or do. Pretty expensive when new though.

Ed

Re: SMITHS "servomotor" 1111SCM-2
 

Now that's interesting...

This indicator has an electronics moule that interfaces with the motors and synchros. I believe it's probably the servo amplifier. It takes signals from the flight data computer and sets the indicator position accordingly.

This module has some parts I've not yet deciphered, pretty much slim black boxes with wire lead protrusions. I have a feeling they may be very early transistors (as there are no valves in the module), but will do some tests to see if there's a PN junction voltage across any of the leads. However, they could be these 'hybrid bricks' mentioned.

I don't intend on using the module as I have no pin-out information on it, so after an age of reverse engineering it, I may end up blowing it up anyway. This is why I want to go straight onto the motors snd synchros with my own circuit, as I have more chance of success as I already have info on them.

Cheers, Scott